1. Field of the Invention
This invention relates to frequency conversion that employs a combination of frequency mixing and multiplication for conversion between a low frequency modulation signal and a radio frequency (RF) signal in the millimeter wave range using a low frequency synthesizer.
2. Description of the Related Art
Millimeter wave radios require conversion of lower-frequency baseband modulation signals to millimeter wave radio frequency (RF) signals for transmission. A baseband modulation signal modulates an intermediate frequency (IF) wave by mixing the modulation signal with the IF wave from a local oscillator to produce a modulated IF signal, the frequency of which is the sum of the baseband frequency and the local oscillator's frequency.
There are two major conventional schemes to convert an IF signal to a millimeter wave RF signal for radio transmission. The first scheme involves direct multiplication of the frequency of the modulated IF signal to a millimeter wave frequency, as shown in FIG. 1. A modulating signal 2 at a low baseband frequency f.sub.mod is mixed with an IF wave 4 at a frequency f.sub.1 in a mixer 6, which produces an upconverted modulated signal 8 at an intermediate frequency f.sub.IF equal to the sum of f.sub.mod and f.sub.1. The IF signal is then fed into a frequency multiplier 10 which increases the frequency of the IF signal by multiplying f.sub.IF by N, resulting in an RF signal 12 at a millimeter wave frequency f.sub.RF. This scheme of direct frequency multiplication is described in U.S. Pat. No. 5,495,255 for a frequency modulation (FM) radar system.
A major disadvantage of the direct frequency multiplication scheme in FIG. 1 for a millimeter wave radio is that the frequency multiplier 10 introduces nonlinearities when the modulated IF signal is multiplied by N, resulting in an RF signal 12 that has a distorted signal shape compared to the modulation signal. Direct multiplication is suitable for modulation schemes that are not amplitude sensitive, such as FM or frequency shift keying (FSK). However, direct multiplication is not suitable for modulation schemes that require linearity, such as amplitude modulation (AM) or high level quadrature amplitude modulation (QAM).
Another conventional scheme to produce an RF signal in the millimeter wave range is to use a millimeter wave synthesizer, as shown in FIG. 2. A baseband modulation signal 14 at a frequency f.sub.mod is mixed with an IF wave 16 at a frequency f.sub.1 in a mixer 18 to produce a modulated IF signal 20 at a frequency f.sub.IF. The IF signal 20 is then mixed with a millimeter wave 22 generated by a millimeter wave frequency synthesizer 24 in a high frequency mixer 26, which produces a modulated millimeter wave signal 28 at a frequency f.sub.RF. This scheme is described in U.S. Pat. No. 5,428,668 for lower RF frequency cellular telephone applications.
The millimeter wave frequency synthesizer 24 in FIG. 2 is very expensive due to hardware complexity required for synthesizing millimeter wave frequencies directly. Moreover, a millimeter wave frequency synthesizer has a limited tuning bandwidth, and therefore is not suitable for wide bandwidth radios. If a multiple number of millimeter wave frequency synthesizers are used to cover a desired tuning bandwidth, the radio would be even more costly.